The present invention relates generally to fluid motors and fluid motor controls and more particularly, to hydrostatic vehicle transmissions which include a fluid pump, a fluid motor, and a control system which enables the speed and direction of the motor to be controlled, which permits the motor to receive an operator-selected output speed setting, which also permits variation of the motor torque and speed to be automatically controlled so as to approach the desired output settings in a highly effective and efficient manner.
In one form, the control system of the invention permits a cam ring or like torque-determining member of the hydrostatic motor to be moved to a desired position as rapidly as operating conditions permit, and permits this position to be maintained during normal vehicle operation. The motor torque automatically adjusts to operating condition demand and the rotational speed is controlled so that the vehicle will maintain a pre-selected speed set by the operator, if conditions permit; otherwise, the motor will adjust itself so as to attempt to approach and maintain this speed, even where this involves automatically adjusting its displacement to reverse motor torque or so-called dynamic braking.
Hydrostatic drives or transmissions are now commonly used in a number of vehicles, including some applications in heavy duty crawler tractors and the like. In a hydrostatic system typically employed, the vehicle engine, ordinarily a diesel or gasoline engine, drives a primary fluid pump unit of the variable displacement type.
A fluid motor also of the variable displacement type is used in association with the variable displacement pump. The control means of the typical system usually produces coordinated movement of both pump and motor displacement controls. Initial motion strokes the pump from a zero displacement position toward a maximum foward or reverse displacement position depending on the vehicle travel direction and speed desired, thereby representing a first operating range. The motor which receives the pump fluid flow and pressure is initially in a maximum displacement condition thereby producing maximum torque through the speed range provided by the zero to maximum pump displacement change. A second operating range is provided in which the coordinated movement control strokes the motor from its normal maximum displacement toward a minimum displacement position. This maneuver further increases the vehicle speed and also reduces the torque output of the motor. The pump/motor stroke control is usually interconnected and manually operated for selecting the desired speed and torque to suit conditions under which the vehicle is operated.
The present invention also uses a variable displacement pump and motor; however, only the motor receives a manually initiated control movement. The pump is of the pressure compensated type well known in the hydrostatic power art in which a normal maximum displacement is automatically reduced toward zero at a predetermined maximum fluid pressure. A nearly constant predetermined pressure is therefore delivered to the motor and used at volume only sufficient to satisfy the combination of speed/torque demand by the motor in accordance with the control setting and vehicle operating conditions.
According to the present invention, the motor or driven portion of the transmission is a radial piston variable displacement type and includes a variable position cam ring in engagement with radially extending end portions of a plurality of pistons.
The fluid displacement caused by piston movement in each rotation is affected by the position of the motor cam ring which is movable within predetermined limits to either side of a center position at which there is no piston stroke and no motor torque. Changing the position of the cam ring to alter the displacement of the pistons for each revolution has been referred to as "stroking" the motor. It is to be understood that the motor will produce a displacement of no volume if the cam ring and the cylinder body containing the pistons are in exact co-axial relation, and that, as the cam ring is offset or disposed eccentrically with respect to the cylinder body, the fluid pressure acts on the pistons, which in turn, act on the cam rings to rotate the cylinder block.
The pressure compensated pump, which is associated with the vehicle engine, is arranged so that it will produce a volume of oil at a pressure up to a predetermined level. If the pump tends to exceed the predetermined output pressure, the internal pump controls are arranged so as to reduce the stroke of the pistons, therefore also reducing the volume output, to an extent sufficient to not exceed the preset pressure; hence, the term, pressure compensated. Somewhat similar considerations apply to the motor, which is essentially also a pump, except that it is normally operated so that it receives a flow of fluid under pressure and produces motion instead of receiving motion and producing a flow of fluid under pressure.
Again referring to a typical vehicle hydrostatic transmission, the engine throttle is positioned at a certain governed speed setting, the pump operates initially at zero displacement and pressure. The output volume of the pump is then selected by the operator within a first speed range and fluid is delivered to the vehicle motor. The driven element or hydrostatic motor receives fluid flow in a variably controlled amount and the power potential of this fluid flow may be translated into a large force or torque at a low rotational speed, or a relatively reduced force or torque at an increased rotational speed by varying the motor displacement.
Hydrostatic transmissions are desirable where load conditions may change continually and it is desired to operate the vehicle engine at a suitable point on its power curve; however, it is possible for the operator to manipulate the controls incorrectly so as to obtain too little vehicle speed, thus losing efficiency, or so as to obtain too much torque, thus overloading or even stalling the engine. Under a constantly changing load, the control lever must often be moved continually for optimum efficiency.
It may also be appreciated that a highly desirable advantage of hydrostatic transmissions, in contrast to hydrodynamic transmissions, is that the output may be readily reversed, simply by shifting of the cam ring or equivalent in the direction opposite the direction required to produce forward motion. In heavy duty equipment, such as earth-moving or other construction equipment where rearwardly directed tractive effort is often required, a significant variation in torque and speed potential in a reverse direction is highly desirable, particularly where this can be accomplished without resort to transmissions using multiple reverse gear sets.
From the foregoing, it will be appreciated that known hydrostatic transmissions have a number of theoretical advantages; however, such transmissions have suffered from certain drawbacks in use and are believed capable of significant further improvement, particularly in the area of motors and control systems therefor.
In view of the disadvantages and drawbacks of existing hydrostatic transmission and control systems, it is an object of the present invention to provide an improved hydrostatic transmission, components thereof, and controls therefor.
Another object is to provide a transmission system including a motor and control which provides variable speed and torque control in both directions of rotation, and which provides for the application of dynamic braking if the vehicle exceeds the speed selected by the operator.
A further object is to provide a transmission system which automatically adjusts the amount of fluid volume utilized at a predetermined pressure level and vehicle speed setting in response to load (vehicle tractive effort demand).
It is another object of the invention to provide a transmission control system in which the operator cannot cause excess power consumption and create excess fluid heating by stalling the vehicle in a condition requiring more torque at a higher speed setting than the traction motor can produce.
Still another object is to provide a transmission system wherein an "arterial" or closed center hydraulic system is used which makes fluid pressure available to perform multiple functions, and which is further arranged so that auxiliary functions may be performed even if full tractive effort is not available for propulsion purposes. In other words the auxiliary functions have priority over the vehicle propulsion function.
Yet another object is to provide a transmission system of reduced mechanical complexity, and particularly a system that does not require that variable displacement pumps and motors be constantly controlled and synchronized.
Another object is to provide a fluid motor with a greatly improved balance of static, dynamic and fluid pressure induced forces.
Still another object is to provide a motor control system which may be operated hydrostatically, electrically or mechanically, and which is useful with different prime power pressure sources and flow control arrangements.
Another object is to provide a transmission which includes a radial piston and cam ring type hydrostatic motor and drive arrangement, including a planetary gear set arranged so that the motor frame rotates about its own axis, carrying with it the cam ring control.
A further object is to provide a control system in which the cam ring control is carried by a rotatable motor frame and in which a cam ring movement is accomplished by rotation of a pinion gear about its own axis, and in which a variable speed arrangement is provided for exact control of cam ring position during rotation of the motor frame.
Another object is to provide a control system having means to prevent the traction motors from receiving an operator signal which, if obeyed, would cause an excessive lowering of hydraulic system pressure.
A still further object is to provide a vehicle control system which, in effect, provides a mechanical memory for the control signals given by the operator and which provides the most rapid response possible to such control signals without overloading the traction motor or causing an undue drop in hydraulic system pressure.
It is another object to provide an improved and highly effective system of pilot or feedback type control of a traction motor in a vehicle drive system by sensing the relation between tractive effort, hydraulic system pressure, and the positioning of the operator control lever.
Yet another object is to provide a transmission system in which the position of the output control for the traction motor is determined by a motor whose rotational speed is in turn controlled by a control pump, and wherein the control pump is responsive to instantaneous conditions of system pressure drop as well as the drive condition selected by the operator.
It is still another object to provide a transmission system in which a control means renders the vehicle propulsion drive function inoperative and hydrostatically locked if the system fluid pressure is below a predetermined safe operating level.
The foregoing and other objects and advantages are achieved in practice by providing a hydrostatic traction motor and control arrangement wherein the output of the traction motor is varied as desired by the operator, if tractive effort requirements are being met by the pump, and motor, and for preventing undue or extremely rapid movements of the traction motor output control where such movements would exceed the power capabilities of the drive system.
The exact manner in which these and other objects and advantages are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiments of the invention set forth by way of example and shown in the accompanying drawings, in which like reference numbers indicate corresponding parts throughout.